1. Field of the Invention
The present invention relates to a frequency detecting circuit, and in particular, to a frequency detecting circuit in which the frequency of signals is detected to output prescribed logic signals.
2. Background of the Related Art
A frequency detecting circuit compares the frequency of inputted signals with a reference frequency to express their magnitude relationship in electrical signals. The input signal and reference signal can be compared using various methods. That is, either the frequencies are directly compared, or the frequency of the input signals is converted into a voltage or current, which is compared with a reference voltage or a reference current. The frequency detection result can be expressed either in analog signals or in digital signals. The analog signals are expressed in a voltage or a current proportional to the detected frequency. The digital signals are expressed by converting the analog signals into the digital signals.
FIG. 1 is a circuit diagram showing a related art frequency detecting circuit. As shown in FIG. 1, clock signals CLK1 to be measured are inputted into a high pass filter consisting of a capacitor C1 and an inductor L. Owing to the high pass filter of the input clock signals CLK1, only clock signals CLK2 higher than a certain frequency are output. The clock signals CLK that have passed through the high pass filter are converted into dc signals by a rectifying diode D to charge a capacitor C2. The dc level of the dc signals from the diode D is proportional to the frequency of the clock signals CLK2, which have passed through the high pass filter. Therefore, the voltage of the capacitor C2 reflects the frequency of the clock signals CLK2.
The voltage of the capacitor C2 is inputted into a non-inverting input terminal of an arithmetic amplifier OP having a feedback resistor R.sub.f. An inverting terminal of the arithmetic amplifier OP receives a reference voltage V.sub.ref, which is formed by two resistors R1 and R2. In accordance with the magnitude relation between the inputted dc signals and the reference voltage, the arithmetic amplifier OP outputs a positive voltage or a negative voltage. Thus, a measurement can be made whether the frequency of the clock signals CLK2 is higher than a frequency represented by the reference voltage.
As described above, the related art frequency detecting circuit has various disadvantages. The related art frequency detecting circuit includes a high pass filter requiring an inductor. It is difficult precisely control circuit characteristics when employing the inductor (e.g., the inductor L) in a fastidious semiconductor integrated circuit. Further, an inductor occupies a large area in the semiconductor integrated circuit, and therefore, the total layout of the frequency detecting circuit is increased. In addition, the related art frequency detecting circuit includes an arithmetic amplifier, which consists of a TTL logic, and therefore cannot be manufactured by the CMOS process. Accordingly, a cost of the circuit is increased.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.